Does any one know where the air screw is on the z250. Mine is always backfiring when Im decelerating so I wanted to know if this would fix it to adjust the air screw.

 

I don't know if that will solve your problem, but I do know its a pain in the butt to get to. I think you will have to remove your carb. to get to it because it is on the underside of it.

 

carb
Its #27 on this picture. There may be a cover over it, that will have to come off.
Usually, if the screw is on the engine side of the carb, it controls mixture and is called a mixture screw. If its on the air cleaner side of the carb, then it controls air, and then its called a air screw. And youre right, if you go a bit richer, the backfire will stop.